The invention relates generally to the field of photography, and in particular to imaging systems producing electronically-derived images that have position dependent blur.
An imaging apparatus, such as a photographic camera or an electronic camera, and in particular its optical assembly, have inherent attributes which can degrade the quality of images captured by the device. In certain cases, such as with single use film cameras or inexpensive digital cameras, it may be economically difficult to avoid usage of inexpensive optics. Unfortunately, such optics possess inherent aberrations that degrade the quality of images formed by the optics. Consequently, it is desirable to compensate for these aberrations in the reproduction process so that final images free of aberrations may be obtained.
For example, a camera system described in U.S. Pat. No. 5,461,440 does not require an expensive optical assembly that is corrected for marginal attenuation (light amount irregularity) and distortion (pincushion and barrel distortion). Instead, the curvature of field data and the light amount irregularity data corresponding to the optical assembly is identified in advance, and stored either in the camera or separately at a downstream scanning and processing station. Either way, the data is linked to the specific camera and then used in subsequent film processing and scanning to correct the image signal for the image quality degradation imparted by the optical assembly.
The image quality of digitally produced prints can be further improved by using an appropriate sharpening or xe2x80x9cedge enhancementxe2x80x9d filter. The appropriate filter depends on the characteristics of the imaging input device (such as a digital camera or a film scanner), the output printer, and the print size. In the prior art, some amount of sharpening is normally performed in the imaging input device, i.e., in the digital camera or the film scanner, and in an output reproduction device, such as a display or a printer. Consequently, it is known for the different parts of the system to have their own sharpening algorithms; in particular, the camera has a hardware sharpening filter, the host computer has user selectable sharpening software, and the printer has a firmware sharpening filter.
The image quality of captured images can be improved by the selection of appropriate filters for the input imaging device and subsequent devices that process the captured images. For instance, in U.S. Pat. No. 4,970,593, the modulation transfer function (MTF) of the uncorrected optical system is measured and an aperture correction function is created from an inverse of the MTF function to correct an image captured through the optical system. Some software packages, such as Adobe Photoshop(trademark), allow the user to select different levels of image sharpening as part of their image processing routines. The amount of sharpening in a printer can sometimes be selected by the user, as can be done in the driver for the Kodak XL7700(trademark) printer, which allows five preset choices of sharpening.
In a typical camera, the image formed at a focal plane (where the film or image sensor is located) can be blurred as a function of proximity to the optical axis of the optical assembly. The further away from the optical axis (normally, the center of the image), the more the image is blurred. The resultant image therefore has an MTF that is a function of radial distance from the center of the image. The problem is exaggerated with images originating from inexpensive cameras, such as single use film cameras. Because of their simple optics or because the film may not be located in the position of best focus throughout the focal plane, single use film cameras tend to have significant sharpness loss with movement away from the optical axis toward the edges of the frame. An optical printing process cannot help the situation, and may instead worsen the situation by introducing the sharpness fall-off of its own optical system.
Especially if they are intended for consumer use, digital cameras, which are inherently more complex and expensive than single use film cameras, must control cost in any way possible. The camera optics is a typical candidate for cost reduction, and position-dependent blurring thus becomes a concern. Despite such image quality concerns, it is usually desirable to provide a finished image file that is corrected for camera-related influences. What is needed is a simple correction for sharpness fall-off that does not require a more complex, or more expensive, optical system, as well as a correction that can be implemented in the processor of a digital camera, or in the downstream scanning and processing of a film system.
In commonly-assigned U.S. Pat. No. 5,696,850, a digital image is produced by a digital camera. This image is operated upon to provide an improved image. This is accomplished by using a sharpening filter which is produced as a function of the system MTF. Although this arrangement produces an improved image, there are still problems with image quality. For example, the image can still suffer from position dependent blur.
An object of the present invention is to provide an improved digital image that eliminates the problems noted above.
Another object of the invention is to provide a modified sharpening kernel which not only sharpens the image, but also corrects for position dependent blur.
These objects are achieved by a method for providing a modified sharpening function which can be used to provide an improved sharpened image from an image having a plurality of pixels by correcting for position dependent blur, comprising the steps of:
(a) providing a sharpening function which is adapted to operate upon signals corresponding to a selected number of pixels;
(b) providing a plurality of values which are a function of the position dependent blur; and
(c) applying the plurality of values to the sharpening function to modify the sharpening function so that after the modified sharpening function is applied to the image, a sharpened image will be provided which has been corrected for the position dependent blur.
An advantage of the present invention is to correct for position dependent blur of image data which can be caused by lens or image sensor phenomena, such as charge-transfer inefficiency, thereby providing improved image quality. The present invention is particularly suitable for use with not only electronic cameras, but can also be used with conventional photographic cameras. It makes use of digital image processing and provides a boost map for modifying a sharpening kernel. This reduces processing steps and provides an effective way for improving image quality.